1. Field of the Invention
The invention relates to electrodynamic machines, and more particularly to inspection and measurement and possible correction of electrodynamic machine shaft radial runout prior to complete machining of the shaft. The present invention is suitable for application to alternating or direct current motor shafts.
2. Description of the Prior Art
It is desirable to operate electrodynamic machines, such as electric motors, within defined vibration parameters. Variation in a shaft's radial runout about its circumference or axial length may impact shaft vibration. Thus motor manufacturers and users often specify shaft runout tolerances and/or ultimate operating vibration parameters, that are measured and confirmed after completion of shaft machining operations.
In many applications it, is also desirable to monitor electrodynamic machine vibration during operation as a potential indicator of operational abnormalities or need for maintenance service (e.g., a breaking rotor bar or shaft bearing in need of lubrication. One known way to monitor operational electrodynamic machine vibration is with at least one proximity probe, such as an eddy current proximity (e.g., radial displacements of a rotating part relative to the probe) to detect vibration values in excess of a predetermined threshold.
Proximity probes or proximity measuring systems can be used for the measurement, monitoring, and/or analysis of axial and/or radial shaft vibration (peak-to-peak displacement amplitude) in electrodynamic machinery. A proximity probe or transducer can be placed in a position defined by a mount. Read-outs from proximity probes, such as via oscilloscope, meter, automated monitoring system, and/or x-y recorder, might not provide an accurate indication of the shaft motion relative to the proximity probe or transducer, because the readings are subject to other influences.
Data read-out provided by the probe can reflect actual movement of the shaft relative to the probe, but also electrical properties of the shaft, and/or inaccuracies generated by the probe itself. The impact of shaft movement can be referred to as “mechanical runout”. The impact of the electrical properties of the shaft can be referred to as “electrical runout”. The impact of the probe's inaccuracies can be referred to as “probe noise”.
Eddy current proximity probes can derive distances, such as proximities, utilizing induced electrical currents in the material of a rotating electrodynamic machine's shaft. Some level of inaccuracy in the values obtained from the probe can be present, however, which can be due to any number of factors, such as instrumentation error, mechanical runout, and/or electrical runout, etc., any of which can vary with measurement location. Electrical runout, often called glitch, can result from variations in electrical properties of the shaft material. Causes for mechanical runout can comprise aberrations in cross-sectional shape and/or axial flatness, etc., bearing hydrostatic effects, bearing hydrodynamic effects, etc.
A known test procedure, to assess inaccuracies comprised in values obtained from the probe, can involve rotating a shaft at a speed below and/or far below a normal operating speed. Such a test procedure can be referred to as a “slow roll” test. A displacement signal that a proximity probe provides during a slow roll test can be called a “slow roll value”.
Rotating equipment can have a maximum specified slow roll value above which the rotating equipment is considered inoperable, because the slow roll, value can mask shaft movement due to dynamically variable vibration. Hence, electrodynamic machine shafts having slow roll values above designated thresholds are considered to be unsuitable for operational service in the past methods and apparatus have been developed to measure total slow roll, value, differentiate contributions to the value attributable to mechanical or electrical runout, values or probe noise, and to remediate shaft properties causing those contributions. For example, measured mechanical runout can be remediated by re-machining the shaft. Electrical runout can be remediated by selective localized heat treatment of the shaft to modify the electromagnetic properties of the shaft material. Probe noise parameters can be isolated and compensated within the proximity probe measuring and monitoring apparatus. Exemplary slow roll measurement and remediation methods and apparatus are shown and described in United States Patent Publications Nos. 2006/0288787 A1, 2006/0288788 A1, and 2010/0024199 A1, the contents of which are incorporated herein by reference.
Unfortunately, despite slow roll measurement and remediation efforts to reduce measured slow roll values below designated specifications, some shafts remain unsuitable for service and are scrapped. Each scrapped shaft represents considerable wasted finished shaft machining cost and effort.
Thus, a need exists in the art for an electrodynamic machine shaft slow roll measurement, testing and remediation method that can be performed on a partially machined shaft. With such a method the manufacturer would have the option to stop further shaft machine operations so as to minimize cost and effort associated with the test failure shaft. The manufacturer, at its option could choose to scrap a slow roll failure shaft, or perhaps recycle a portion of the shaft for a smaller frame motor. Alternatively if the slow roll tests on the partially machined shaft are successful, the manufacturer may complete the remainder of the machining operations with confidence that the shaft should meet completed shaft final slow roll measurement specifications.